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Quasi-phase matched (QPM) second-order nonlinear optical processes in compound semiconductors are attractive
for frequency conversion because of their large nonlinear susceptibilities and their mature fabrication processes
that permit monolithic integration with pump lasers and other optical elements. Using quantum well intermixing
(QWI), we have fabricated domain-disordered QPM (DD-QPM) waveguides in GaAs/AlGaAs superlattices and
have previously demonstrated continuous-wave (CW) Type-I second-harmonic generation (SHG) and pulsed
Type-II SHG. CW experiments were complicated by Fabry-Perot resonances and thermal bistability. Experiments
using a 2-ps pulsed system were affected by third-order nonlinear effects, group-velocity mismatch (GVM), and
poor spectral overlap with the conversion bandwidth. A better evaluation of the conversion efficiency may,
however, be determined by using longer pulses in order to avoid these complications. By this, the effective CW
conversion efficiency and χ(2) modulation can be ascertained. In this paper, we demonstrate SHG in DD-QPM
waveguides with reduced parasitic effects by using 20 ps pulses. The waveguide structure consisted of a core
layer of GaAs/Al0.85Ga0.15As superlattice into which QPM gratings with a period of 3.8 μm were formed using
QWI by As2+ ion implantation. For a Type-I phase matching wavelength of 1583.4 nm, average second-harmonic
(SH) powers produced were as high as 2.5 μW for 2 ps pulses and 3.5 μW for 20-ps pulses. At low input powers,
the SHG average power conversion efficiency of the 2-ps system was more than 10 times larger than the 20 ps
system. As power was increased, the SH power saturated and conversion efficiency decreased to nearly equal
to the 20-ps system which remained consistent over the same power range. This is attributed to a reduction in
third-order nonlinear effects, a smaller pulse spectral width that overlaps better with the conversion bandwidth,
and less pulse walkoff for the 20-ps pulses. Thus, by using 20-ps pulses over 2-ps pulses, we achieved similar
output SH powers and potentially higher SH powers are possible since there was no observed saturation at high
input power.
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